A conventional electro-photographic image forming apparatus forms an electrostatic latent image by exposing a photosensitive medium to light by scanning a laser beam on the photosensitive medium, and forms a toner image by selectively supplying toner to the electrostatic latent image on the photosensitive medium from a developing roller that is either spaced apart from or in contact with the photosensitive medium.
Such a conventional electro-photographic image forming apparatus typically requires a laser scanning apparatus for scanning a laser beam on the photosensitive medium. The laser scanning apparatus requires highly accurate optical arrangement, and thus tends to be expensive. Accordingly, an apparatus that can replace the laser scanning apparatus is desirable. To that end, a print head is being developed to include a number of light emitting devices, such as light emitting diodes (LED), organic light emitting diodes (OLED), or inorganic electroluminescence (EL), in sufficient number as to correspond to a number of pixels for concurrent forming at least a portion of the electrostatic latent image on the photosensitive medium.
For example, shown in FIG. 1 is an illustration of an example conventional print head 19. The print head 19 includes light emitting points 18 arranged in a uniform pitch. The light emitting points 18 are formed on a transparent board 12, and include an opaque metal electrode 24 forming a cathode used as a common electrode, an organic light emitting layer 22 emitting a beam and a transparent electrode 20 forming an anode for addressing each light emitting point 18.
However, since a beam emitted from a light emitting device, such as an OLED or an inorganic EL, has a wide divergence angle, a crosstalk between two beams emitted from neighboring light emitting points 18 may occur, causing the image quality to suffer, e.g., due to an unintended pixel being irradiated. In an attempt to focus the beams emitted from each light source on the corresponding pixel of the photosensitive medium 10, a micro-lens array including convex lenses 16 each corresponding to the respective light emitting point 18 are provided. However, even when the beams are focused by using the convex lenses 16, a chromatic aberration may be generated because of the wide spectrum of wavelength associated with some of the light emitting devices, such as, e.g., an OLED or an organic EL, and it is difficult to form an accurate image on the photosensitive medium 10 with the small depth of focus.
Further, the non-planar nature of the micro-lens array of convex lenses 16 makes it difficult to fabricate the micro-lens array directly on the surface of the light emitting device, and thus may complicate the manufacturing process of the print head by requiring additional fabrication steps and/or elements, such as a spacer.